You can use math.stackexchange.com for this today. It's frowned upon to just ask an exercise from a book w/o even trying to solve it, but if you show that you made an effort but got stumped, or if you show your solution and ask if it's correct, people will gladly help you out.

I think academics would get pretty annoyed :) Most of my professors just use the exercises from the textbook for homework, usually on the assumption that you can't find the answers online.

I'm sure they would, but I'm more concerned about people in my shoes. University tuition has gotten so expensive these days that I think we need solid alternatives...and that they can use some of that fancy tuition money to write their own exercises, if they don't trust students to do their own work.

Or they could just trust the students. At my university the honor code such a big deal that they let students take closed-book tests at home.

Not a problem - Boas has the answers for every 2nd question - for tutorials, we would be asked to do the ones without answers.

Maybe DennisP's idea could do the same thing - only post answers to the odd-numbered questions. Of course, DennisP's scheme would only work for books that actually have decent end-of-section questions, unless people made up extra questions as well ...